Railway signal system with speed determined movement detector

ABSTRACT

A railway signal system for detecting a train approaching a railroad grade crossing, track section or the like transmits in the track a periodically interrupted carrier wave signal the amplitude of which is attenuated by an approaching train effecting a variable shunt across the track, and an automatic gain control receiving a highly selective filtered input regulates the track signal current to provide a wide window or range of track conditions over which the system operates effectively for train detection. A receiver responsive to the received track signal picks up a signal relay unless an approaching train, broken rail, extreme track ballast or system malfunction is detected. Moreover, an automatic pulse-height control varies system sensitivity to detect rapidly approaching trains at long distances.

United States Patent [1 1 Geiger Dec. 30, 1975 RAILWAY SIGNAL SYSTEM WITH SPEED DETERMINED MOVEMENT DETECTOR [21] Appl. No.: 458,172

Related U.S. Application Data Continuation-impart of Ser. No. 348,944, April 9, 1973, Pat. No. 3,850,390.

ll/l97l /1972 Campbell 246/125 Risely v. 246/ Primary ExaminerLeo Friaglia Assistant ExaminerDavid M. Mitchell Attorney, Agent, or Firm-Donnelly, Maky, Renner & Otto [57] ABSTRACT A railway signal system for detecting a train approaching a railroad grade crossing, track section or the like transmits in the track a periodically interrupted carrier wave signal the amplitude of which is attenuated by an approaching train effecting a variable shunt across the track, and an automatic gain control receiving a highly [52] US. Cl. 246/130; 246/34 CT; 246/128 51 int. cm B61L 23/04 selewve filtered mput regulates the rack sgnal 581 Field of Search 246/125, 34 R, 34 CT, 128, FP Pwvde Wmdow range i 246/130 R 40 34 A 28 C dltlOl'lS over which the system operates e ectivey or tram detection. A receiver responsive to the received [56] References Cited track signalgl picks ppla sigtnal reltay ulplgsslant approach;

ing ram, ro en ar ex reme rac a as o sys e UNITED STATES PATENTS malfunction is detected. Moreover, an automatic fi y 246/34 R pulse-height control varies system sensitivity to detect 86S 3,529,150 9/1970 Coupin rapidly approaching trams at long distances. 3,610,920 lO/l97l Frielin haus 246/128 30 Claims, 5 Drawing Figures L 1| oer: on.

l42 r94 9 2 s5 92 86 LOSS OF HIGH/LOW smm'r SIGNAL oer. DISABLG. osrzcroR CIRCUIT no L 930 l3 I2 2 Q M X i o i a,

US. Patent Dec. 30, 1975 Sheet 1 of3 3,929,307

Hue F23: m "-0 mmOJ US. Patent Dec.30,1975 Sheet2of3 3,929,307

Q24 6- 20mm JOmFZOO US. Patent Dec. 30, 1975 Sheet 3 of3 3,929,307

RAILWAY SIGNAL SYSTEM WITH SPEED DETERMINED MOVEMENT DETECTOR BACKGROUND OF THE INVENTION This patent application is a continuation-in-part of my copending patent application Ser. No. 348,944, filed Apr. 9, 1973, for Railway Signal System With Speed Determined Movement Detector, now US. Pat. No. 3,850,390, and is assigned to the same Assignee.

This invention relates to a fail-safe railway signal system for detecting an approaching train and particularly to such a system having automatic gain control, automatic sensitivity control, broken rail detection, and system malfunction detection capabilities.

Prior art railway signal systems for detecting approaching trains have used various techniques to compensate for train approach distance and speed to achieve minimum down time during which a signal device is activated or a control signal is generated to provide an indication, for example, at a grade crossing or at another track section, when a train is approaching, existing in, or leaving a railway crossing area or a specific track section. The control signal may be coupled to a control system including, for example, a computer for automated train control, or may be used to operate a relay, signal lights or the like.

Several existing signal systems respond only to a train within the island between the transmitter and receiver connections to the track, requiring long islands to provide train detection within a safe time, and the lengthy island increases the difficulty of system installation. Other devices respond to train approach speed, but do not include variable sensitivity features, such devices often requiring plural systems operating at different frequencies for achieving a minimum safe down time of the signal device.

One disadvantage with prior art railway signal systems is that without variable sensitivity, a train consisting of only a single car or engine may suddenly accelerate after approach time prediction to put the engine almost in the crossing before gate actuation, and another disadvantage is the relatively long ring-by time occasioned by a lengthy island. Also, prior art systems are incapable of effective operation over a wide range of conditions, such as varying track ballast, or in the presence of spurious track signals. Moreover, prior art systems using DC amplifiers require checking pulses at regular periods, and authenticity of a train indicating signal is lost during a five or six period until a self-check pulse is transmitted.

SUMMARY OF THE INVENTION Briefly described, the invention comprises a system capable of responding to variations in the lumped impedance value of a track caused by the moving shunt effect of an approaching train and the system is effectively operable over a wide range of dynamic track ballast conditions by virtue of the wide window between high and low track signal voltages during which the system is accurately responsive .to approaching train speed and distance. The system includes a transmitter which provides to atrack a pulse modulated AC carrier wave signal regulated by an automatic gain control. A receiver having a highly selective input filter to receive the track signal includes a motion detecting portion and an astable multivibrator responsive to the output of the latter for generating an output signal to pick up or to drop a signal relay, such output signal also being capable for use as a control signal for application to further apparatus, such as a computer, signaling system, or the like. Moreover, an automatic pulse height control circuit responsive to the motion detecting portion controls the sensitivity of the system by varying the magnitude of the pulses modulating the AC carrier wave signal; and high and low signal detectors, also responsive to the motion detecting portion, are coupled to the multivibrator output to provide the same system output indicative of an approaching train whenever the track signal detected by the receiver is higher or lower than the limits defining a predetermined range.

Accordingly, a primary object of the invention is to provide a railway signal system improved in the noted respects.

Another object of the invention is to provide a variable sensitivity movement detector for detecting a rapidly approaching train at long distance and a relatively slowly approaching train at close distance.

An additional object of the invention is to regulate automatically the track signal, power level, and particularly the current thereof, transmitted by a railway signal system.

A further object of the invention is to provide movement detection, broken rail detection, island control, variable sensitivity and maximum integrity features in a railway signal system.

Still another object of the invention is to detect accurately an approaching train on. a railroad track which undergoes a wide variation in effective impedance.

Still an additional object of the invention is to provide a railway signal system operable over a wide range or window of track signal voltages to accurately detect an approaching train in order to have minimum safe down time, such system also being self-checking and providing an indication of occurrence of track signal voltages beyond either end of such window.

These and other objects and advantages of the instant invention will become more apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described in the specification and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but several of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS In the annexed drawings:

FIG. I is a schematic electric circuit diagram, partially in block form, of a railway signal system in accordance with the invention;

FIG. 2 is a schematic electric diagram of the motion detecting and the high and low signal detecting portions of the receiver;

FIG. 3 is a graph of an interrupted or pulse modulated carrier wave AC signal generated by the transmitter of the railway signal system for application to th track as the track signal;

FIG. 4 is a graph of a DC signal with am impresse AC pulse, which is developed in the motion detecting portion; and

DESCRIPTION oF THE PREFERRED EMBODIMENTS The railway signal system transmitter and receiver are connected to a section of railroad track, for example, at a railway grade crossing or on a specific track section in a block signal arrangement, defining an island between such connections. The interrupted or pulse modulated carrier wave track signal generated by andcoupled to the track from the transmitter tends to vary with variations in the effective lumped impedance of the track ballast; therefore, an automatic gain control accurately responsive to the current applied to the track functions to maintain the track signal current relatively constant over a wide range of ballast conditions. Variations in the ballast may be caused, for example, by changing weather conditions, changing track integrity, the shunt effect of an approaching train, and the like. Moreover, the track signal voltage is monitored by high and low signal detectors in the receiver for defining the limits of a voltage window, whereby track signal voltages within the window make the system accurately speed and distance determining and voltages outside the window result in an indication at the system output, for example, by the dropping or de-energization of the relay operated by the system.

Variations in the effective track ballast impedance caused by the moving shunt effect of an approaching train so as to vary the track signal causes operation of the motion detecting portion to indicate an approaching train within a minimum safe down time of the signal relay. Since it is desirable to detect a rapidly approachingv train at a greater distance from the island than a slowly approaching train and since it is desirable to detect a slowly approaching train only sufficiently in advance of the island to provide a minimum safe down time, the system incorporates an automatic sensitivity control responsive to the track signal voltage to vary the percent modulation of the carrier wave signal applied to the track, whereby minimum modulation effects a maximum power track signal which travels a maximum distance in the tracks for detection of trains at long approachdistances and an increase in percent modulation reduces the effective track signal power to reduce the sensitivity of the railway signal system making the same then accurately responsive to train approach speed.

The railway signal system is fail-safe, is self-checking at a rate of approximately five times per second, is relatively insensitive to noise, automatically regulates the track signal current and effective track signal power, is accurate to detect motion as slow as several feet per second on a close approach and to detect rapid motion on a long approach, responds to broken rail conditions within or beyond the island, and provides an indication when the track signal falls below a predetermined. level.

Referring now initially to FIGS. 1 through 4 in the drawings, wherein like reference numerals refer to like elements in the several figures, the railway signal system is' generally indicated at l. A transmitter 2 generates a pulse modulated carrier wave signal and is transformer coupled at 3 via an impedance matching network generally indicated at 4 and surge protection apparatus 5, such as a lightning arrester or the like, to

a connection point or tic points 6 on the railroad track 7. A receiver 8 is also coupled to the track 7 at a connection point or tie points 9 via a surge protection device 10, impedance matching network 11, transformer l2, and adjustable potentiometer 13. The output 14 of the railway signal system is transformer coupled at 15 to a full wave bridge rectifier 16 to pick up or energize or to drop or de-energize a railway signal relay 17 or the like. Thus, with all portions of the system 1 functioning properly, tolerable track ballast conditions and no train being detected within the monitored approach to the island 18 defined between the system connection points 6, 9 to the track 7 or existing in the island itself, the relay 17 will be energized. Alternatively, should the opposite of one of the aforementioned conditions exist, the relay 17 will be de-energized.

In the transmitter 2 a conventional tone generator 20 generates at its output 21 a carrier wave signal that is applied to one input of a gate and buffer amplifier 22 in which a pulse modulating signal is combinedwith the carrier wave signal to provide at the gate and buffer amplifier output 23 a signal similar in shape to that graphically illustrated in FIG. 3, such signal comprising a high frequency component 24 which is modulated to fall within the envelope defined by the pulse modulating signal 25. The gate and buffer amplifier includes a conventional potentiometer offset circuit 26 for adjusting the maximum amplitude of the pulse modulated carrier wave signal provided at the output 23. The pulse modulating signals are generated in a conventional pulser circuit 27 including, for example, a unijunction transistor oscillator, the output 28a from which is made adjustable by the inclusion of a sensitivity control circuit 29; and pulses of a magnitude determined by the amount of conduction in a transistor 30 are provided on line 28b. The tone generator 20, gate and buffer amplifier 22 and pulser 27 are described in more detail in my above-identified copending patent application, and the tone generator, which is preferably of the electro-mechanical type including a conventional reed oscillator, driving and feedback circuits, is also described in detail in my US. pat. No. 3,740,550, issued June 19, 1973, and assigned to the same Assignee as the instant application.

The output 23 from the gate and buffer amplifier 22 is coupled to the input 31 of a track driver amplifier 32 consisting of one or more stages for driving the transformer 3. The impedance matching network also includesthe primary winding of the transformer 33, the secondary winding of which is coupled to the input of a highly selective electro-mechanical filter 34, which also receives power from the terminals 35, 36 coupled across the zener diode and capacitor voltage regulation circuit 37. The filter includes an electro-mechanical reed oscillator 38, and a driving signal for the reed oscillator is provided across the parallel connected resistor and capacitor 39, with an output from the reed oscillator being taken at 40 to control conduction in a transistor 41. The reed oscillator 38 is tuned to the same frequency as the reed oscillator in the tone generator 20 so as to be driven only by a signal at the transformer 33 having a frequency of the carrier wave signal; therefore, the filter 34 is effectively immune to spurious track signals feeding back to the impedance matching network 4 and the transistor 41 collector output will be representative only of the track signal current generated in the transmitter 2.

The filter 34 provides a signal on the' potentiometer 42 at the input of an automatic gain control arrangement 43 which maintains the track signal current generated by the transmitter 2 at a relatively constant level over a wide range of variations in the track ballast. The automatic gain control 43 includes a track power control amplifier 44 having a coupling capacitor 42a at its input, which consists of one or more stages and reeeives a power input from the terminals 35, 36 coupled across the zener diode and capacitor voltage regulation circuit 47, and the output from the track power control amplifier is transformer coupled at 48 to a full wave bridge rectifier 49, the output of which is connected across an RC filter circuit 50. The output from the track power control amplifier 44 is proportional to the track signal current, and, therefore, the DC voltage appearing at the terminal 51a is also proportional to the track signal current. A line 52 couples the terminal 51a to the control portion 53 of the automatic gain control 43 via a zener diode 54 and diode 55 to the base bias circuit 56 of a transistor 57. The emitter of the transistor 57 is coupled to the negative power terminal, and the collector thereof is coupled by a resistor 58 and diode 59 to a point 60 intermediate a pair of isolation capacitors 61, 62 coupled between the output 23 of the gate and buffer amplifier 22 and the input 31 of the track driver amplifier 32.

The transmitter 2 and automatic gain control 43 are adjustable so that the control portion 53 of the latter assures a relatively constant track signal current at, for example, a 200 milliamp level. The adjustment is made by substantially centrally adjusting potentiometer 42 to effect midrange conduction in the transistor 57, and the potentiometer offset circuit 26 in the gate and buffer amplifier 22 is adjusted until the track signal current measured, for example, at the point 6 achieves the desired level. Thereafter, any increase or decrease in track signal current will be reflected via the transformer 33 and filter 34 into the automatic gain control circuit 43 effecting greater conduction in the transistor 57 to lower the track signal current or lesser conduction in that transistor to increase the track signal current.

The pulse modulated carrier wave track signal received at the receiver 8 is filtered in a highly selective input filter 60 tuned to the frequency of the carrier wave signal, and the filtered track signal is amplified in receiver amplifier 61, which receives a solid state regulated power input at 62 and comprises one or more stages for providing first and second output signals on the respective lines 63, 64. The line 63 is connected to the input of the motion detecting portion 65, and the line 64 is connected to an island control circuit 66, which includes one or more amplifier stages, transformer, rectifiers, filters, and the like, as shown, for example, in my above-identified patent application, whereby the island control provides on the output line 67 a DC voltage whenever the receiver 8 receives a track signal, i.e. when no train is present in the island 18. On the other hand, the line 67 goes to ground potential when a train is present in the island.

The motion detecting portion 65 includes a movement detector driver 70, which has an output transformer coupled at 71 to a full wave bridge rectifier 72; and an output 73 from the latter is coupled to the input ofa wave shaping circuit 74, which in turn is coupled to the movement detector 75. The movement detector driver, wave shaping circuit 74, movement detector 75 and negative slope detector 76 to which the output 77 of the latter is connected are described in more detail below as well as in my above-identified copending patent application. A capacitor 78 responsive to the sig nal appearing at the output 73 of the bridge rectifier 72 provides a substantially average DC voltage at the out put 73, such DC voltage being proportional to the track signal voltage.

The signal at the output 73 from the bridge rectifier 72 is applied as a wave shaping circuit 74, which upon receipt of a proper track signal by the receiver normally generates an output signal E illustrated in FIG. 4 as a DC voltage v with a periodic DC pulse impressed thereon, the pulse corresponding in frequency to that of the pulse generated by the pulser 27 in the transmitter 2. Each time the impressed pulse goes negative simulated movement is detected in the movement detector 75, and as the pulse again recovers in the positive direction movement detection ceases. A light emitting diode 79 in the movement detector flashes periodically to indicate proper functioning of the system 1 when no train is detected. When actual train movement is detected the movement detector output 77 goes to a constant DC level and the negative slope detector 76 remains in saturation. Moreover, a loss of shunt detector 80, which is connected to the point 73, provides a further input to the movement detector on the line 81 in order to preclude a loss of motion detection thereby upon a temporary loss of shunt of an approaching train, which passes over along a rusty rail, bounces above the track or the like. Moreover, the line 67 from the island control 66 is coupled by way of the loss of shunt detector to an input 82 of the negative slope detector for providing power to the same, whereby the presence of a train in the island 18 effectively grounds the line 67 cutting off power to the negative slope detector and eliminating an AC output to the pulse coupling transformer 83 connected to the power transistor output 84 and to the system output circuit portion 85.

The secondary winding of the pulse transformer 83 is coupled to a storage capacitor 85a at the input of a conventional astable multivibrator 86. The capacitor 85a provides power to one-half of the multivibrator, and a signal applied to the terminal 51b, which is coupled by a line not shown to the terminal 51 a, charges another capacitor 87 to provide power to the other half of the multivibrator. The output of the multivibrator is taken on the line 88, which is coupled via a coupling capacitor 88a to the input of the railway signal relay driver amplifier 89. The relay driver amplifier receives a power input from the terminals and voltage regulating circuit 90 and provides an AC output signal to the transformer 15 whenever both sides of the multivibrator 86 are powered, such AC signal being rectified in the bridge rectifier, 16 and used to pick up the relay 17. If the relay driver 89 does not receive an AC input, no signal will be coupled across thetransformer 15, and the relay 17 will be dropped.

Referring back to the output point 73 from the bridge rectifier 72, a resistor 91 couples such output to a line 92, the portion 92a of which is connected to a high/ low signal detector 93. The high signal detecting portion of the detector 93 is responsive to occurrence of a track signal voltage greater than a predetermined level, which would be likely to occur, for example, when a rail of the track 7 is broken substantially increasing the effective track impedance; and the low signal detecting portion responds to a track signal voltage below a predetermined minimum, which may be caused by extremely low impedance track ballast condition or the presence of an undetected train proximate the island 18. In the event of a high or low signal detection by the detector 93, an output is provided on the line 94 to eliminate any AC signal on the line 88 from reaching the relay driver 89, and the relay 17 will, therefore, be dropped. A disabling circuit 93a for the high/low signal detector 93 is coupled to the output of the loss of shunt detector 80 for disabling the low signal detecting portion upon occurrence of a leaving train.

Moreover, a portion 92b of the line 92 is connected to the base bias circuit 95 of a transistor 96 in the automatic control portion of the sensitivity control circuit 29. A pair of resistors 97a, 9711 are connected across a regulated power 98 for the sensitivity control circuit, and the collector and emitter of the transistor 96 are connected across the latter resistor, whereby the amount of conduction in the transistor 96 determines the voltage level at the collector thereof. Moreover, a manually adjustable portion of sensitivity control circuit 29 includes a potentiometer 99, and the signal tapped from the potentiometer is combined with the signal at the collector of the transistor 96 and provided to control conduction in the transistor 30. Thus, the potentiometer 99 may be centrally adjusted, for example, and if the track signal voltage is relatively high, the voltage at the output 73 of the bridge rectifier 72 will also be high to cause maximum conduction in the transistor 96, minimum conduction in the transistor 30 and, therefore, minimum modulation of the carrier wave signal, whereby the system 1 then operates in a maximum sensitivity mode. In such mode the track signal is at maximum power and travels a maximum distance in the track for detection of rapidly approaching trains at a maximum safe distance from the island. Alternatively, as the track signal voltage decreases, for example, due to an approaching but undetected train, the signal at point 73 will decrease, conduction in the transistor 96 will decrease, conduction in the transistor 30 will increase, and, therefore, the percent modulation of the carrier wave signal will increase, whereby the sensitivity of the system 1 decreases in relation to the track distance over which the track signal travels. In the latter mode the large size pulses received at the receiver 8 assure accurate and effective train detection based on the train approach speed and distance from the island 18.

Referring now more particularly to FIG. 2, portions of the receiver 8 are shown in greater detail than in FIG. 1, and like reference numerals refer to corresponding parts in both figures. In the motion detecting portion 65 the signal on line 63 from the receiver amplifier is amplified in the movement detector driver 70, transformed, rectified and filtered and applied as a substantially average DC level signal at the point 73 for application to the wave shaping circuit 74, loss of shunt detector 80, and via resistor 91 to the pulse height control on line 92b and to the high/low signal detector on the line 92a. The RC network 100 in the wave shaping circuit 74 normally produces at the node or terminal 101 the signal having a wave shape as indicated in FIG. 4. A DC blocking differentiating capacitor 102 couples the wave shaping circuit to the Darlington pair amplifier 103 at the base bias circuit thereof, which includes resistors 104, 105 and a potentiometer 106 for determining the sensitivity of the movement detector 75. The effective gain of the amplifier 103 is variable,

determined by the amplitude of the DC part of the E signal normally blocked by the capacitor 102 and the proportional pulse thereof. The collector output of the amplifier 103 is connected to a resistor 107 and to the base of a transistor 110 through a voltage dropping resistor 111. The transistor 110 is connected between the negative line 112 and through a resistor 113 and light emitting diode 79 to the positive line 114, and the transistor 110 serves as the output of the movement detector 75.

The negative slope detector 76 is normally maintained at saturation, and an input applied on the line 77 thereto due to the pulse of the E signal driving the amplifier 103, which in turn drives transistor 110, periodically drives the negative slope detector out of saturation to effect an AC signal output at the power transistor 84 which drives the primary winding of the pulse coupling transformer 83. A train in the island 18 eliminates the voltage on the line 67 which goes to ground disabling the transistor 84.

The loss of shunt detector 80 receives an input signal on the line 80a through an RC network coupled to control a transistor 131, which is connected between the positive line 114 and a timing circuit 132. The output 133 from the timing circuit is connected by the line 81 to the base input of a further Darlington pair amplifier 134, which is connected between a resistor 135 and the emitter output of the Darlington pair amplifier 103. A resistor 136 connects the emitters of both such amplifiers to the negative line 1 12. Thus, a control signal to the base of the amplifier 134 causes the emitters of both amplifiers to go positive to assure cut off of the amplifier 103.

The timing circuit 132 is described in detail in my above-identified copending patent application and, further, the disabling circuit portion 140 of the loss of shunt detector 80, which includes a transistor 141 coupled by the line 142 to the secondary winding of the pulse transformer 83, is also described in detail in such patent application.

In operation of the movement detecting portion 65, the partially filtered signal at the point 73 is shaped in the wave shaping circuit and provided as the E signal to the capacitor 102, the transistor 131 in the loss of shunt detector being maintained non-conducting at this time. In an exemplary embodiment of the invention, when no train is within the beginning of approach shunts on the track 7 or within the range of track through which the track signal is effectively transmitted, the E signal has a 40 volt DC level v and a proportional 2.5 volt peak to peak pulse. When a train is at a certain location between a beginning of approach shunt and the island 18, effecting a shunt across the tracks and reducing the effective track ballast seen by the transmitter 2 and receiver 8, the E signal is reduced, for example, to a 20 volt DC level and a 1.25 volt pulse.

The differentiating capacitor 102 normally blocks the DC part of the E signal and provides through the pulses impressed on such DC part a constant selfchecking of the railway signal system, for example, at a rate of approximately five times per second. The capacitor 102 may be considered in a 0 or charged state, i.e. it charges back to zero state condition at a constant charging rate. Thus, each time an E, pulse goes negative, simulated motion is detected by the movement detector 75 and the amplifier 103 is cutoff; and as the pulse recovers in the positive direction, the capacitor 102 recharges and the amplifier 103 conducts. When a continued slow drop of the DC part of the E signal occurs, for example, due to a slowly approaching train far from the island 18, the differentiating capacitor maintains itself only a slight amount away from the zero state, drawing very little current from its recharging circuit, the base biasing network 104, 105, 106 of the amplifier 103, and the E pulses will effect an AC output from such amplifier.

A train on the track 7 within the beginning of approach shunt and traveling toward the island 18 is a traveling shunt that reduces the track signal as well as the E signal at the node 101, such signal reductions being non-linear with respect to train distance from the island due to the non-linearity of the accumulated track ballast impedance in the approach. It has been found, for example, that signals having frequencies from to 65 Hz exhibit some semblance of linearity of attenuation over a given length of rail, whereas higher frequency signals from 300 to 3000 Hz are substantially non-linear throughout the entire approach.

In one embodiment when the DC part of the E signal is at 40 volts with a 2.5 volt pulse, an approaching train causing a drop of the E signal in excess of approximately 0.4 volt per second will bias the amplifier 103 to cut off preventing it from passing the pulses through the succeeding stages and allowing the relay 17 (FlG. 1') to drop. Similarly, when the E signal is at 20 volts with a 1.25 volt pulse, a 0.2 volt per second drop in the DC part will maintain the amplifier cut off. Once cut off the amplifier 103 is maintained cut off due to a continuing E drop since the capacitor 102 cannot recharge instantaneously and the base of the amplifier 103 is held negative due to the traveling shunt effect of the approaching train.

The railway signal system 1 becomes increasingly more sensitive to E drop as the train approaches the island because the E pulses require less of an E,,, signal drop to bias the amplifier 103 to cut off. Moreover, since the track ballast inpedance is non-linear, a train three thousand feet from the island must effect, for example, a reduction of the E signal at a rate of approximately 0.4 volt per second whereas a train only several hundred feet from the island effects a reduction of the E signal at a rate of 0.01 1 volt per second.

An AC output from the movement detector 75 on the line 77 cuts off the normally saturated active transistor in the negative slope detector 76 when such AC signal reaches its negatively sloped portion, and the output from the negative slope detector is, therefore, a square wave used to drive the transistor 84 to produce an AC signal in the transformer 83 for powering the one half of the multivibrator 86. Since the negative slope detector is operated alternately at saturation and at cut off, it is relatively immune to noise and provides circuit isolation and uniform regulation of signals applied to the multivibrator 86. Also, the negative slope detector reduces ring-by time as the train leavesthe island 18 because it is then operated in saturation and cut off, the AC output therefrom being a strong broad square wave signal which assures energization of the transformer 83.

In the loss of shunt detector 80 the-transistor 131 is normally maintained cut off due to the blocking effect of the capacitor in the RC network 130. When an approaching train has caused the E signal to drop to a point where motion has been detected by the movement detector 75, the transistor 150 is maintained cut off; however, a rapid increase in E voltage due to a temporary loss of the train shunt effect causes the tran- 10 sistor 131 to conduct charging the capacitors in the timing circuit 130, which bias the amplifier 134 to conduction to assure maintaining the amplifier 103 cut off. When a train has entered the island, any charge stored in the timing circuit 132. will discharge along the line 67 to the island control circuit.

A train leaving the island causes a relatively slow increase in E voltage, although the pulses thereof are quickly recognized by the amplifier 103, which again initiates an AC signal in the transformer 83 to result in a power signal again being applied to the multivibrator 86. The AC signal at the secondary of the transformer 83 is also applied by a line 142 to cause periodic conduction in the transistor 141., which discharges any accumulated charge within the timing circuit 132, thus disabling the loss of shunt detector as the train leaves the island.

The high/low signal detector '93 includes a high signal detecting portion 150, which provides the system 1 with broken rail detection capability, whereby occurrence of a broken rail on the track 7 causes the track signal voltage to increase substantially and an output from the high level detecting portion will be indicative of such condition. The input from the line 92a to the high level detecting portion is provided through a diode 151, resistor 152, and potentiometer 153 to the negative line 154. The potentiometer 153 is manually ad justable to determine the upper limit of the track signal voltage window over which the system 1 will operate without providing a broken rail or high signal indication, and the wiper of the potentiometer is connected via a zener diode 155 and a resistor 156 to control conduction in a transistor 157, which is connected at its emitter to the negative line 154. The collector of the transistor 157 is connected to the base of a transistor 158 in the low signal detecting portion 160, which receives an input from the line 92a via the diode 161, zener diode 162, manually adjustable potentiometer 163, and resistor 164, which is coupled to the negative line 154. The potentiometer 1163 may be adjusted to determine the lower limit of the window, above which the transistor 158 is conductive and below which such transistor is non-conductive. An output portion for the high and low signal detecting portions includes an RC circuit 166 coupled between the collector output of the transistor 158 and the base inputs of a pair of transistors 167, the emitters of which are connected to the negative line 154 and the collectors of which are connected by the line 94 to the multivibrator 86 output, shown in FIG. 1, or to the multivibrator output as shown in FIG. 5.

In operation of the high and low signal detecting portions 150, 160, when the track signal voltage falls within the predetermined window, the transistor 157 will be non-conductive and the transistor 158 will be conductive so as to preclude any bias signal from effecting conduction in the transistors 167. Any signal appearing at the multivibrator output line 88 will be undisturbed. However, if the track signal voltage falls below the window, the transistor 158 will become nonconductive, or if the track signal voltage exceeds the maximum limit of the window, the transistor 157 will conduct to assure cut off of the transistor 158. In either of the latter two cases, a signal from the positive line 168 will effect conduction in the transistors 167, which draws any signal at the multivibrator output 88 via the line 94 to the negative line 154, causing the relay 17 to drop.

The disable circuit 93a for the high/low signal detector 93 receives an input on the line 170 coupled to a pair of resistors 171, 172, which are in turn connected to the line 142 to receive power whenever an AC signal is being coupled through the pulse transformer 83 from the motion detecting portion 65 to the astable multivibrator in the output portion of the receiver. The line 170 is connected via a diode 173 to the base input of a transistor 174, the emitter of which is connected to the negative line 154 and the collector of which is connected via a resistor 175 to the positive linel68. The collector is also connected via a pair of diodes 176, 177 and an RC circuit 178 to the gate electrode of an SCR 179, which has its anode and cathode connected between the collector of the transistor 158 and the negative line 154. Due to the integrating effect of the capacitor 85 at the input to the multivibrator 86, the signal on the line 142 will be a DC level with an impressed AC pulse; and when train movement has not been detected, such signal is propagated through the resistors 171, 172 to maintain conduction in the transistor 174. As long as the transistor 174 is conducting, the SCR 179 will not conduct-However, upon train detection the voltage on line 42 will go to zero, the transistor 174 will cease conducting, the SCR 179 will receive a gating signal causing conduction therein precluding conduction in the transistor 167 and capacitor 178:: will charge. The train leaving the island will allow the track signal voltage to rise, again effecting conduction in the transistor 174; however, in order to disable the low signal detecting portion 160 until the track signal voltage has risen above the minimum window level, the capacitor 178a in the RC circuit 178 discharges through the diode 177 to maintain conduction in the SCR 179. Thus, ring-by is not increased by the low signal detecting portion.

In some-installations, it may be desirable to eliminate the effect of the low signal detecting portion, especially at grade crossings proximate a railroad yard wherein trains or cars thereof are often stored at locations near the crossing, maintaining the track signal voltage below the minimum window level. By shunting the terminals 179, the transistor 158 will be maintained conductive, unless the high signal detecting portion 150 becomes active causing conduction in the transistor 157. Thus, the low signaldetecting portion is inactive and the high signal detecting portion remains active.

Turning now more particularly to FIG. 5, a modified form of a railway signal system is illustrated generally at 200. The various elements in the system 200 which are identical with those illustrated in FIGS. 1 and 2 are designated by like reference numerals.

In FIG. 5 the transmitter 2 includes a tone generator 20, gate and buffer amplifier 22, pulser 27, sensitivity control circuit 29, automatic gain control portion 201, and track driver amplifier 32, the output from which is transformer coupled at 3 to a connection point 6 on the railraod track 7. The current applied to the track is monitored by the transformer 33, the secondary of which is connected to the highly selective filter 34, which provides an output via the coupling capacitor 42a to the input of the track power control amplifier 44, which includes at least one transistor amplifier stage 44a. The output from the track power control amplifier is transformed at 48, rectified at 49 and filtered at 50 to provide a DC voltage at the terminal 51a, and such voltage is applied on the line 52 via a zener diode 54, diode 5S, resistor 202 and RC circuit 203 to a ground connection 204. The voltage drop between the resistors 202, 203 is applied to the base bias circuit 56 of the transistor 57, which is connected at its emitter to a negative terminal and at its collector via a resistor 58 and diode 59 to the point 60 between the isolation capacitors 61, 62 to couple the output from the gate and buffer amplifier 22 to the input of the track amplifier 32. Thus, by adjusting the appropriate potentiometers at the track power control amplifier 44 and gate and buffer amplifier 22 the track signal current may be fixed for a given track ballast condition, the transistor 57 then conducting in approximately midrange to reduce the output signal from the gate and buffer amplifier to some extent before application thereof to the track driver amplifier 32. As the track signal current increases or decreases, conduction in the transistor 57 will correspondingly increase or decrease to maintain the track signal current substantially constant.

The receiver 8 is connected at 9 to the track 7 at the other end of the island 18 from the transmitter, and the receiver amplifier 61, movement detecting portion 65, including the movement detector driver 70, wave shaping circuit 74, movement detector 75, negative slope detector 76 and loss of shunt detector 80, as well as the island control circuit 66 are identical with those elements described above with reference to FIG. 1.

The output from the negative slope detector is coupled by the transformer 83 to the output circuit portion 210 at capacitor 210a at the power input of the astable multivibrator 211. Thevoltage at the positive side of the capacitor 210a is normally a DC level with an impressed AC pulse, and such DC voltage is provided via the resistors 212, 213 to provide power to both halves of the multivibrator 211. The output from the multivibrator is taken on the line 214 for application to the input of the railway signal relay driver 215, a first stage of which being illustrated in dashed outline including a transistor 216 and a coupling capacitor 217. The terminal 51b, which is coupled to the terminal 51a by a connection not shown, is connected via resistors 218, 219 to provide power to the collector of the transistor 216.Thus, if the voltage at the terminal 51a goes to zero, the transistor 216 loses its power supply and no signal will be transmitted via the coupling capacitor 217 to the remaining portions of the relay driver 215, and the relay 17 will be dropped.

A high signal detecting portion 220 receives an input along the line 92a from the output 73 of the bridge rectifier 72. Resistor 221 and potentiometer 222 are connected between the line 92a and the negative line 154, and the manually adjustable wiper arm 223 of the potentiometer 222 is connected via a line 224 to the base of transistor 44a in the track power control amplifier 44. Thus, upon occurrence of a high track signal voltage beyond the limits of the voltage window, the transistor 44a will be maintained conducting regardless of the output on the filter 34, and no AC signal will be transmitted through the transformer 48, whereby the voltage at the terminals 51a, 51b goes to zero and the relay 17 will drop. As indicated above the high track signal voltage normally occurs when a track rail breaks, and, therefore, the high signal detector serves as a broken rail detector for the system 200.

The low signal detecting portion 230 includes an input from the line 92a via the resistor 231 and manually adjustable potentiometer 232 the latter being connected to the negative line 154; and the adjustable wiper of the potentiometer 232 is coupled to the base bias circuit 233 of the transistor 234, which receives collector power from the positive terminal 235 via a resistor 236. The collector of the transistor 234 is connected via an RC circuit 237 to the base inputs of a pair of transistors 238, which have their emitters connected to the negative line 154 and their collectors connected to the output 214 from the multivibrator 211. In operation of the low signal detecting portion 230, a track signal voltage falling within the window provides for conduction in the transistor 234, whereby the transistors 238 will be cut off. In the event the track signal voltage drops below the window lower than the predetermined level, which can be adjusted by the potentiometer 232, the transistor 234 will stop conducting, and the transistors 238 will conduct to eliminate the multivibrator output signal from the line 214 at the input to the relay driver 215, thus causing the relay 217 to drop. The disabling circuit 93a is identical to that illustrated in FIG. 2 and functions in the same manner to disable the low signal detector upon occurrence of a train leaving the island.

It should now be appreciated that the railway signal system operates effectively in a fail-safe mode over a wide range of track conditions to provide indications of a train approaching or existing in an island or of a defect in the system itself. The system also has an increased sensitivity with increased proximity to the island and an automatically variable sensitivity to detect rapidly approaching trains at long distances, is constantly self-checking and continuously monitors track conditions.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising:

means for generating an AC signal, said means for generating including means for coupling the same to said track, whereby said AC signal is transmitted in said track;

means for automatically regulating the current level of said AC signal; in accordance with variations in the track ballast; and

receiver means coupled to said track for receiving said AC signal from said track, said receiver means including means for detecting said AC signal to effect production of said system output signal in response to changes in said AC signal occurring due to such an approaching train.

2. The system of claim 1, further comprising electromechanical filter means coupled to said means for coupling and tuned to the frequency of said AC signal for providing an input signal to said means for automatically regulating, said input signal being proportionally representative of said AC signal generated by said means for generating and transmitted in said track.

3. A system as set forth in claim 2, further comprising an amplifier coupled to said electro-mechanical filter means; and an output circuit portion including an astable multivibrator having at least a first power input coupled to said receiver means; whereby when said astable multivibrator receives power, it is free running and when it does not receive power, it is not free running and said system output signal indicative of an approaching train is produced; and said amplifier being coupled to said output circuit portion to control production of said system output signal.

4. The system of claim 3, said output circuit portion comprising an output'AC amplifier coupled to receive an input from said astable multivibrator, said first-mentioned amplifier being coupled to provide a power input to said output AC amplifier, whereby elimination of the first-mentioned amplifier output signal disables said output AC amplifier and said system output signal is then produced.

5.'The system of claim 3, said astable multivibrator having a second power input coupled to said amplifier; whereby when both power inputs of said astable multivibrator receive power, said multivibrator is free running, and when at least one of said power inputs does not receive power, said multivibrator is not free running and system output signal indicative of an approaching train is produced.

6. The system of claim 1, said means for automatically regulating comprising an automatic gain control responsive to the average current level of said AC signal generated by said means for generating.

7. The system of claim 6, said means for generating comprising plural amplifier stages, the output from one stage being coupled to provide an input signal to the next stage, and an output of said automatic gain control being coupled to said amplfier stages for controlling the magnitude of input signal from said one amplifier stage to said next amplifier stage.

8. The system of claim 6, said automatic gain control including an 'electro-mechanical filter tuned to the frequency of said AC signal, said electro-mechanical filter being coupled to said means for coupling and hence providing an input signal to said automatic gain control in response to said AC signal.

9. The system of claim 6, said means for generating including an amplifier, and said automatic gain control being coupled to said amplifier to control the effective gain of said amplifier.

10. The system of claim 1, said means for generating comprising means for generating an AC carrier wave signal and means for periodically modulating said AC carrier wave signal, whereby said AC signal transmitted in said track by said means for generating is a modulated carrier wave signal; and further comprising means for controlling the sensitivity of said system comprising means responsive to the magnitude of said AC signal received at said receiver means from said track for adjusting the percent modulation of said AC carrier wave signal, thereby varying the effective power of said AC signal transmitted in said track by said means for generating.

11. The system of claim 10, said receiver means including a bridge rectifier for developing a DC signal proportionally representative of the voltage level of said AC signal received at said receiver from said track; and said means for adjusting comprising a transistorized circuit responsive to said DC signal for varying the percent modulation of said carrier wave signal.

12. The system of claim 1, said AC signal transmitted in said track comprising a track signal, and said means for generating normally generating said AC signal such that the voltage level of said track signal upon receipt at said receiver means is within a predetermined range; said system further comprising means for sensing when said track signal voltage level is outside said range, said means for sensing being coupled in said receiver means to effect production of said system output signal indicative of an approaching train upon such detection.

13. The system of claim 12, said means for sensing comprising high signal detector means for sensing received track signal voltage levels above said range and low signal detector means for sensing received track signal voltage levels below said range, and means for disabling said low signal detector means upon occurrence of a train leaving the system tie points to the track.

14. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising:

means for transmitting a track signal in said track,

said means for transmitting including means for generating an AC carrier,wave signal, means for periodically modulating said carrier wave signal, and means for coupling said means for generating to said track, whereby said track signal comprises a modulated carrier wavesignal;

receiver means coupled to said track for receiving said track signal, said receiver means including means responsive to said track signal for producing said system output signal in response to changes in said track signal occurring due to an approaching .train; and means responsive to. the magnitude of said track signal received at said receiver means for automatically varying the sensitivity of said railway signal system, said means for varying comprising means for controlling the percent modulation of said carrier wave signal in response to the magnitude of said received track signal, thereby adjusting the effective power level of the track signal transmitted in said track.

15. The system of claim 14, said receiver means comprising a bridge rectifier for producing a DC signal proportionally representative of the voltage level of said track signal received by said receiver means from said track; said means for controlling comprising a first transistorized circuit responsive to said DC signal, and a second transistorized circuit responsive to said first transistorized circuit for contolling the percent modulation of said AC carrier wave signal.

16. The system of claim 15, further comprising means for manually controlling conduction in said second transistorized circuit.

17. The system of claim 16, said means for generating an AC carrier wave signal comprises a tone generator and said means for periodically modulating comprises a unijunction transistor and capacitor oscillator for developing modulating pulses to modulate said AC carrier wave signal, said second transistorized circuit being coupled between said tone generator and oscillator for controlling the size of said modulating pulses.

18. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising:

means for generating an AC signal, said means for generating including means for coupling the same to said track, whereby said AC signal is transmitted in said track;

means for monitoring the current level of said AC signal generated by said means for generating and transmitted in said track, said means for monitoring including an electromechanical filter tuned to the frequency of said AC signal, and said means for monitoring producing an output signal indicative of the current level of said AC signal generated by 1 said means for generating and transmitted in said track;

receiver means coupled to said track for receiving said AC signal from said track, said receiver means including means for detecting said AC signal, said means for detecting providing an AC output indicative of such detection; and

an output circuit portion means being coupled to said receiver means and to said means for monitoring for developing said system output signal indicative of an approaching train when said AC signal is not detected at said means for detecting or the current level of said AC signal falls below a predetermined minimum level.

19. The system of claim 18, said output circuit portion means comprising an astable multivibrator having first and second power input circuits, said first input circuit being coupled to said means for detecting and said second input circuit being coupled to said means for monitoring. 7

20. The system of claim 18, said output circuit portion means comprising an amplifier having at least one power input, said means for monitoring being coupled to said at least one power input, whereby when the power level of said AC signalfalls below a predetermined level said amplifier is disabled and said system output signal indicative of an approaching train is pro- 'duced.

21. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising:

means for transmitting a track signal in said track, said means for transmitting including'means for generating an AC signal and means for coupling said means for generating to said track;

receiver means coupled to said track for receiving said track signal, said receiver means including an output circuit portion means for developing said system output signal upon occurrence of an approaching train;

said means for generating normally generating said AC signal such that the voltage level of said track signal upon receipt at said receiver means is within j a predetermined range; and

means for sensing when such voltage level is outside said range, said means for sensing being coupled in said receiver means to effect production of said system output signal indicative of such an approaching train when such voltage level is outside said range.

22. The system of claim 21, said means for sensing comprising high signal detector means for sensing voltage levels above said range and low signal detector means for sensing voltage levels below said range, and means for disabling said low signal detector means upon occurrence of a train leaving the system tie points to the track.

23. The system of claim 22, said means for sensing comprising a transistorized output'circuit coupled to said receiver output circuit portion means for effecting production of said system output signal upon energization of said transistorized output circuit.

24. The system of claim 23, wherein said low signal detector means comprises a first transistorized circuit normally energized when such voltage level is within said range to maintain de-energization of said transistorized output circuit, and said high signal detector means comprises a second transistorized circuit normally de-energized when such voltage level is within said range, said second transistorized circuit being coupled to said first transistorized circuit, whereby when such voltage level is above said range said second tran sistorized circuit is energized to effect de-energization of said first transistorized circuit and energization of said transistorized output circuit.

25. The system of claim 24, said means for disabling comprising an SCR coupled between the output of said first transistorized circuit and the input from the latter to said transistorized output circuit, means for energizing said SCR upon detection of an approaching train thereby eliminating energization of said transistorized output circuit, means for precluding energization of said SCR when no train is detected, and timer circuit means for assuring continued conduction in said SCR for a time period upon occurrence of a leaving train.

26. The system of claim 25, said receiver means comprising a motion detection portion normally producing an AC output upon receipt by said receiver means of said track signal when no train has been detected, said AC signal normally being coupled to said disabling circuit.

27. The system of claim 21, further comprising means for monitoring said AC signal generated by said means for generating, an output from said means for monitoring being coupled to said receiver means output circuit portion means for controlling the same such that when the power level of said AC signal falls below a predetermined level said output circuit portion means is controlled to develop said system output signal indicative of an approaching train.

28. The system of claim 27, said means for sensing comprising high signal detector means for sensing such voltage levels above said range, said high signal detector means being coupled to said means for monitoring to disable the same, whereby upon occurrence of such voltage level above said range said high signal detector disables said means for monitoring effecting control of said output circuit portion means to develop said system output signal indicative of an approaching train.

29. The system of claim 28, said receiver means comprising means for producing a DC signal proportionally representative of said track signal voltage level, said high signal detector means comprising a resistance divider circuit, and said means. for monitoring comprising an AC amplifier, whereby upon occurrence of a track signal voltage level above said range, said high signal detector biases said AC amplifier to saturation so that no output signal is produced by the latter.

30. The system of claim 28,, said means for sensing further comprising low signal detector means for producing a first output signal when said track signal voltage level is within said range and a second output signal when said track signal voltage level is below said range, said low signal detector means being coupled to said receiver means output circuit portion means such' that application of said second output signal to said receiver means output circuit portion means causes the latter to develop said system output signal indicative of an approaching train, and further comprising means coupled to said receiver means for disabling said low signal detector means upon occurrence of a leaving train. 

1. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising: means for generating an AC signal, said means for generating including means for coupling the same to said track, whereby said AC signal is transmitted in said track; means for automatically regulating the current level of said AC signal; in accordance with variations in the track ballast; and receiver means coupled to said track for receiving said AC signal from said track, said receiver means including means for detecting said AC signal to effect production of said system output signal in response to changes in said AC signal occurring due to such an approaching train.
 2. The system of claim 1, further comprising electromechanical filter means coupled to said means for coupling and tuned to the frequency of said AC signal for providing an input signal to said means for automatically regulating, said input signal being proportionally representative of said AC signal generated by said means for generating and transmitted in said track.
 3. A system as set forth in claim 2, further comprising an amplifier coupled to said electro-mechanical filter means; and an output circuit portion including an astable multivibrator having at least a first power input coupled to said receiver means; whereby when said astable multivibrator receives power, it is free running and when it does not receive power, it is not free running and said system output signal indicative of an approaching train is produced; and said amplifier being coupled to said output circuit portion to control production of said system outPut signal.
 4. The system of claim 3, said output circuit portion comprising an output AC amplifier coupled to receive an input from said astable multivibrator, said first-mentioned amplifier being coupled to provide a power input to said output AC amplifier, whereby elimination of the first-mentioned amplifier output signal disables said output AC amplifier and said system output signal is then produced.
 5. The system of claim 3, said astable multivibrator having a second power input coupled to said amplifier; whereby when both power inputs of said astable multivibrator receive power, said multivibrator is free running, and when at least one of said power inputs does not receive power, said multivibrator is not free running and system output signal indicative of an approaching train is produced.
 6. The system of claim 1, said means for automatically regulating comprising an automatic gain control responsive to the average current level of said AC signal generated by said means for generating.
 7. The system of claim 6, said means for generating comprising plural amplifier stages, the output from one stage being coupled to provide an input signal to the next stage, and an output of said automatic gain control being coupled to said amplfier stages for controlling the magnitude of input signal from said one amplifier stage to said next amplifier stage.
 8. The system of claim 6, said automatic gain control including an electro-mechanical filter tuned to the frequency of said AC signal, said electro-mechanical filter being coupled to said means for coupling and hence providing an input signal to said automatic gain control in response to said AC signal.
 9. The system of claim 6, said means for generating including an amplifier, and said automatic gain control being coupled to said amplifier to control the effective gain of said amplifier.
 10. The system of claim 1, said means for generating comprising means for generating an AC carrier wave signal and means for periodically modulating said AC carrier wave signal, whereby said AC signal transmitted in said track by said means for generating is a modulated carrier wave signal; and further comprising means for controlling the sensitivity of said system comprising means responsive to the magnitude of said AC signal received at said receiver means from said track for adjusting the percent modulation of said AC carrier wave signal, thereby varying the effective power of said AC signal transmitted in said track by said means for generating.
 11. The system of claim 10, said receiver means including a bridge rectifier for developing a DC signal proportionally representative of the voltage level of said AC signal received at said receiver from said track; and said means for adjusting comprising a transistorized circuit responsive to said DC signal for varying the percent modulation of said carrier wave signal.
 12. The system of claim 1, said AC signal transmitted in said track comprising a track signal, and said means for generating normally generating said AC signal such that the voltage level of said track signal upon receipt at said receiver means is within a predetermined range; said system further comprising means for sensing when said track signal voltage level is outside said range, said means for sensing being coupled in said receiver means to effect production of said system output signal indicative of an approaching train upon such detection.
 13. The system of claim 12, said means for sensing comprising high signal detector means for sensing received track signal voltage levels above said range and low signal detector means for sensing received track signal voltage levels below said range, and means for disabling said low signal detector means upon occurrence of a train leaving the system tie points to the track.
 14. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, compriSing: means for transmitting a track signal in said track, said means for transmitting including means for generating an AC carrier wave signal, means for periodically modulating said carrier wave signal, and means for coupling said means for generating to said track, whereby said track signal comprises a modulated carrier wave signal; receiver means coupled to said track for receiving said track signal, said receiver means including means responsive to said track signal for producing said system output signal in response to changes in said track signal occurring due to an approaching train; and means responsive to the magnitude of said track signal received at said receiver means for automatically varying the sensitivity of said railway signal system, said means for varying comprising means for controlling the percent modulation of said carrier wave signal in response to the magnitude of said received track signal, thereby adjusting the effective power level of the track signal transmitted in said track.
 15. The system of claim 14, said receiver means comprising a bridge rectifier for producing a DC signal proportionally representative of the voltage level of said track signal received by said receiver means from said track; said means for controlling comprising a first transistorized circuit responsive to said DC signal, and a second transistorized circuit responsive to said first transistorized circuit for contolling the percent modulation of said AC carrier wave signal.
 16. The system of claim 15, further comprising means for manually controlling conduction in said second transistorized circuit.
 17. The system of claim 16, said means for generating an AC carrier wave signal comprises a tone generator and said means for periodically modulating comprises a unijunction transistor and capacitor oscillator for developing modulating pulses to modulate said AC carrier wave signal, said second transistorized circuit being coupled between said tone generator and oscillator for controlling the size of said modulating pulses.
 18. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising: means for generating an AC signal, said means for generating including means for coupling the same to said track, whereby said AC signal is transmitted in said track; means for monitoring the current level of said AC signal generated by said means for generating and transmitted in said track, said means for monitoring including an electromechanical filter tuned to the frequency of said AC signal, and said means for monitoring producing an output signal indicative of the current level of said AC signal generated by said means for generating and transmitted in said track; receiver means coupled to said track for receiving said AC signal from said track, said receiver means including means for detecting said AC signal, said means for detecting providing an AC output indicative of such detection; and an output circuit portion means being coupled to said receiver means and to said means for monitoring for developing said system output signal indicative of an approaching train when said AC signal is not detected at said means for detecting or the current level of said AC signal falls below a predetermined minimum level.
 19. The system of claim 18, said output circuit portion means comprising an astable multivibrator having first and second power input circuits, said first input circuit being coupled to said means for detecting and said second input circuit being coupled to said means for monitoring.
 20. The system of claim 18, said output circuit portion means comprising an amplifier having at least one power input, said means for monitoring being coupled to said at least one power input, whereby when the power level of said AC signal falls below a predetermined level said amplifier is disabled and said system output signal indicatIve of an approaching train is produced.
 21. A railway signal system for producing an output signal indicative of a train on a track approaching the system tie points to the track, comprising: means for transmitting a track signal in said track, said means for transmitting including means for generating an AC signal and means for coupling said means for generating to said track; receiver means coupled to said track for receiving said track signal, said receiver means including an output circuit portion means for developing said system output signal upon occurrence of an approaching train; said means for generating normally generating said AC signal such that the voltage level of said track signal upon receipt at said receiver means is within a predetermined range; and means for sensing when such voltage level is outside said range, said means for sensing being coupled in said receiver means to effect production of said system output signal indicative of such an approaching train when such voltage level is outside said range.
 22. The system of claim 21, said means for sensing comprising high signal detector means for sensing voltage levels above said range and low signal detector means for sensing voltage levels below said range, and means for disabling said low signal detector means upon occurrence of a train leaving the system tie points to the track.
 23. The system of claim 22, said means for sensing comprising a transistorized output circuit coupled to said receiver output circuit portion means for effecting production of said system output signal upon energization of said transistorized output circuit.
 24. The system of claim 23, wherein said low signal detector means comprises a first transistorized circuit normally energized when such voltage level is within said range to maintain de-energization of said transistorized output circuit, and said high signal detector means comprises a second transistorized circuit normally de-energized when such voltage level is within said range, said second transistorized circuit being coupled to said first transistorized circuit, whereby when such voltage level is above said range said second transistorized circuit is energized to effect de-energization of said first transistorized circuit and energization of said transistorized output circuit.
 25. The system of claim 24, said means for disabling comprising an SCR coupled between the output of said first transistorized circuit and the input from the latter to said transistorized output circuit, means for energizing said SCR upon detection of an approaching train thereby eliminating energization of said transistorized output circuit, means for precluding energization of said SCR when no train is detected, and timer circuit means for assuring continued conduction in said SCR for a time period upon occurrence of a leaving train.
 26. The system of claim 25, said receiver means comprising a motion detection portion normally producing an AC output upon receipt by said receiver means of said track signal when no train has been detected, said AC signal normally being coupled to said disabling circuit.
 27. The system of claim 21, further comprising means for monitoring said AC signal generated by said means for generating, an output from said means for monitoring being coupled to said receiver means output circuit portion means for controlling the same such that when the power level of said AC signal falls below a predetermined level said output circuit portion means is controlled to develop said system output signal indicative of an approaching train.
 28. The system of claim 27, said means for sensing comprising high signal detector means for sensing such voltage levels above said range, said high signal detector means being coupled to said means for monitoring to disable the same, whereby upon occurrence of such voltage level above said range said high signal detector disables said means for monitoring effecting control of said output circuit portion means to develop said system output signal indicative of an approaching train.
 29. The system of claim 28, said receiver means comprising means for producing a DC signal proportionally representative of said track signal voltage level, said high signal detector means comprising a resistance divider circuit, and said means for monitoring comprising an AC amplifier, whereby upon occurrence of a track signal voltage level above said range, said high signal detector biases said AC amplifier to saturation so that no output signal is produced by the latter.
 30. The system of claim 28, said means for sensing further comprising low signal detector means for producing a first output signal when said track signal voltage level is within said range and a second output signal when said track signal voltage level is below said range, said low signal detector means being coupled to said receiver means output circuit portion means such that application of said second output signal to said receiver means output circuit portion means causes the latter to develop said system output signal indicative of an approaching train, and further comprising means coupled to said receiver means for disabling said low signal detector means upon occurrence of a leaving train. 